Allow project PYTHONPATH updating for all drag & drop operations on fold...

...ers.

Provide the PydevPackageExplorer with its own drag & drop service that chooses PyDev drop
assistants over all others (instead of the other way around, which is the default). By so
doing, allow project PYTHONPATH updates for all cases of dragging & dropping folders,
including cases where a non-source folder containing source folders is moved.

------------------

Here's another attempt at fixing the problem with dragging parent folders. I did what I could to override the default sorting algorithm that chooses ResourceDropAdapterAssistant over PyResourceDropAdapterAssistant, so now PyDev operations are given higher priority for PyDevPackageExplorer drag operations. If you'd prefer another approach please let me know.